1. Technical Field
The present invention relates to a light guide plate and an area light source device. Specifically, the present invention relates to an area light source device that is used as a backlight for a liquid crystal display device and a light guide plate constituting the area light source device.
2. Related Art
Nowadays, there is increasing a demand for a low profile of an area light source device with the progress of a low-profile mobile device provided with the area light source device. In order to thin the area light source device, it is necessary to thin a light guide plate. However, even if the flat light guide plate can be thinned, there is a limitation to a reduction of a height of a light source including an LED. Therefore, in the case that the thin, flat light guide plate is used, the height of the light source is larger than a thickness of an end surface (a light incident surface) of the light guide plate, the light source disposed opposite the light incident surface of the light guide plate is projected from an upper surface of the light guide plate. When the light source is projected from the light guide plate, whole light emitted from the light source is not incident to the light incident surface of the light guide plate, but the light leaks partially to the outside to degrade light use efficiency.
Use of a light guide plate, in which a light introduction part larger than a thickness of a flat light guide plate body is provided at an end of the light guide plate body and an inclined surface inclined from a maximum thickness point of the light introduction part toward the end of the light guide plate body is provided in the light introduction part, has been proposed. For example, PCT International Publication No. WO2010/070821 and PCT International Publication No. WO2008-153024 disclose the area light source devices in each of which the light guide plate is used.
FIG. 1 illustrates an example of an area light source device 11 in which the light guide plate including the light introduction part thicker than the light guide plate body is used. A light guide plate 13 includes a light guide plate body 14 having a substantially even thickness and a wedge-shaped light introduction part 15. A deflection pattern or a diffusion pattern is formed in a rear surface of the light guide plate body 14, and a lenticular lens 16 is formed on a surface. An inclined surface 17 is formed in the light introduction part 15. The inclined surface 17 is inclined from a point having the maximum thickness of the light introduction part 15 toward an end of the light guide plate body 14. The thickness of an end surface (a light incident surface) of the light introduction part 15 is larger than a height of the light source 12. In the area light source device 11 in which the light guide plate 13 is used, the thickness of the end surface of the light introduction part 15 is larger than the height of the light source 12, whereby the light introduction part 15 efficiently takes in the light emitted from the light source 12. The light taken in by the light introduction part 15 is guided to the light guide plate body 14 and spread in a flat manner, and the light is reflected by the deflection pattern or the diffusion pattern, and output to the outside from a light exit surface of the light guide plate body 14. At this point, a directional pattern of the light output from the light exit surface is spread by the lenticular lens 16. Therefore, in the area light source device having the above structure, not only the light use efficiency of the light source can be improved but also the low profile of the area light source device can be achieved.
However, for the area light source device 11 in which the inclined surface 17 is provided in the light introduction part 15 as illustrated in FIG. 1, the light traveling in a direction oblique to an optical axis of the light source 12 in the light introduction part 15 is reflected by the inclined surface 17 and spread in a horizontal direction (a width direction of the light guide plate) by an arrow in FIG. 1, and an angle formed between the light and the optical axis of the light source 12 increases when the light guide plate body 14 is viewed from above. As a result, the light horizontally spread by the inclined surface 17 leaks from a side surface of the light guide plate 13, or the light is horizontally incident to the lenticular lens 16 and leaks from the lenticular lens 16, whereby the degradation of the light use efficiency or the degradation of luminance evenness is generated by a light quantity loss.
FIG. 2 is a perspective view of the area light source device disclosed in PCT International Publication No. WO2010/070821. In an area light source device 21 in FIG. 2, plural V-groove light leakage prevention patterns 22 parallel to one another are provided in the inclined surface 17 of the light introduction part 15. The light leakage prevention pattern 22 reduces the light leakage from the inclined surface 17 to improve the light use efficiency. At the same time, the light reflected by the light Leakage prevention pattern 22 is easily diffused in the horizontal direction compared with the case that only the inclined surface 17 is provided without providing the light leakage prevention pattern 22. Therefore, the horizontal spread of the light increases, and the light leaks further easily from the side surface of the light guide plate 13. Additionally, in the case that optical patterns, such as the lenticular lens, are provided in the upper surface or the lower surface of the Light guide plate 13, the light leaks easily from the optical pattern. As a result, the degradation of the light use efficiency or the degradation of the luminance evenness is also generated by the light quantity loss.